Infraacoustic/electric fish fence

ABSTRACT

An immaterial fish fence is based on a combination of low frequency mechanical vibrations and synchronously modulated electric fields, where fish approaching the fence will be given at the same time fear reactions and directional information by mechanical vibrations, and in addition will feel pain due to the electric field. The fish will then turn and swim away. The fence is implemented by means of columns positioned side by side, each comprising a number of low frequency transducers suspended above each other, each column being suspended in a float. Each column also has two electrical conductors to which a high voltage can be delivered. Thus, synchronized fields of both acoustic and electric type can be generated between and around the columns.

BACKGROUND OF THE INVENTION

The present invention relates to fish fence primarily of an immaterialtype, based on subsea use of low frequency mechanical (infra acoustic)vibrations in conjunction with an electric field where two such signalsare modulated synchronously. The infraacoustic field combined with theelectrical field create anxiety reactions in fish. Fish also will feelpain caused by the electric field.

Usually, a fish fence is material, i.e. sometimes a net is used actingas an obstruction for fish exceeding a certain size.

However, in certain cases such a material fence constitutes a problem,e.g. by being a hindrance to other sea traffic, by the fact that thefence is exposed to fouling, as well as by the fact that it is difficultto alter the mesh in order to vary the size of the fish it is desirableto have pass through the fence. Consequently, a different type of fenceis needed, for example an immaterial "energy fence", also called"non-physical" fence, of a type to irritate/frighten and cause pain, soas to make fish turn back when about to swim into the fence.

Previously there have been conducted experiments using different formsof both sound and electrical fences, however not in combination. In themajority of experiments where sound has been used, sound pressure hasbeen used, in the sense that the actual sound pressure is used tofrighten away the fish or to make the fish stay behind or inside animaginary barrier.

These experiments to generate sound pressure in water have all beenconducted within the hearing range of the fish (i.e. the range 50Hz-approximately 2000 Hz), audible to fish via otolith and/or swimmingbladder, but these methods have proved to be not very effective, for thesimple reason that the fish gets used to the sound.

Additionally, several experiments have been performed with electricalfences with an aim at establishing an electrical field in the water.Experiments have been conducted with different frequencies and pulsewidths in an alternating electric field and, generally, it can be statedthat extremely varied results were achieved. The best results wereobtained when using a very short pulse width and frequencies of 50 Hz.In this case a barrier efficiency of approximately 80% was achieved, butsmall fish were nevertheless able to swim rather easily through thefence, and larger fish were killed. The reason for these problems isthat the fish have no organs to indicate from which direction theelectric field comes, this resulting in that the fish, upon entering theelectric field, will feel pain, but will not be able to detect thereason therefor. Usually it will continue swimming into the field, whereit will either be killed or may succeed in passing through (sometimesinjured), depending on its physical size.

New investigations show that fish exposed to infraacoustic particlemovements (both the acceleration and velocity of the water particles areimportant) can detect such movements almost down to a frequency of 1 Hz(experiments have been performed down to 3 Hz) with the use of side lineorgans. In addition to detecting the particle movement (the particleacceleration), the fish is also able to detect the direction.

Fish have such a sensor system as a predator warning, and experimentsshow that fish exposed to infra acoustic accelerations get spontaneousreactions of fear (under 20 Hz).

Experiments with an infraacoustic fence with a dipole type accelerationpattern can produce relatively good results, but the result isnevertheless too dependant on the surroundings, i.e. factors like stresslevel (traffic), conditions of food/light and threshold of fear (due topossible pedators in the surroundings).

From U.S. Pat. No. 2,146,105 is previously known electrodes which createelectrical dipole fields, but no additional stimulus of the acoustictype to impart fear and direction information to the fish, appearstherein.

A combined system, however, is known from U.S. Pat. No. 2,709,984butthis combination relates to light signals and electric influence of thefish from a plurality of electrodes suspended in the water. Correctly,it is stated in such publication that the fish needs a "direction"indicator in addition to the electric shocks sensed, in order to realizein which direction it must swim to avoid the unpleasant effect. However,important differences exist between light and sound regarding suchdirection indication, as light is a positive stimulus and hence the fishis dependent on training to achieve the desired effect (i.e. the fishmust collate a flash of light with a simultaneous electric shock).

However, it would be far more advantageous if an immediate response forthe fish could be obtained without any prior learning process.

SUMMARY OF THE INVENTION

The present invention tries to achieve an effective solution to theproblem regarding the generation of an immaterial barrier in water inorder to stop the approach of the fish, where the sound pressure andintensity of an electric field are not of great importance, but on thecontrary infraacoustic particle acceleration in conjunction with a veryshort and intense electric field modulated synchronously with theinfraacoustic field. There is no learning problematics involved, as thefish already from the first try will swim away from the electric field.

It appears namely that when the fish enters the infraacousticacceleration vector field, it exhibits a fear reaction, and at the sametime it feels pain due to a voltage drop across the body as a result ofthe electric field.

Since the fish is able to detect the acoustic field (which is aligned inparallel with the electric field as discussed below, together with thedirection thereof, pain provided by electricity will be perceived asassociated with the acoustic field and the direction-found source of thefield, and the fish will turn and swim away from what it considers to bea position which is the cause of the pain.

The invention is implemented by a fish fence based on under water use oflow frequency mechanical vibrations combined with an additionalstimulus, and is particularly characterized by a plurality ofelectromagnetic low frequency transducers for mechanical vibrations,suspended in vertical columns beneath floats positioned in regular rowsin or below the water surface, such transducer configurationconstituting a grid, and a corresponding plurality of pairs ofparallel-connected electric conductors, each conductor in a pair beingpositioned close to and along each respective side of a verticaltransducer column. The additional or further stimulus an electric fieldfrom the conductors, modulated in synchronism with the mechanicalvibrations from the transducers. The transducers establish the acousticfield, and the electrodes on each side of the transducers establish theelectric field.

The transducers in each column (together with the electrodes) arearranged to oscillate in phase with each other and in opposite phasewith the nearest neighboring column in the row (it is also possible touse the alternative that the transducers in a column oscillate in theopposite phase with the next nearest neighboring column. In this way anacoustic field is raised together with an electric field as a row ofsingle dipole configurations in the water between the transducercolumns.

In a preferred embodiment of the invention, the transducers areconnected to a drive voltage generator the time behavior of which isadapted by a Fourier series to provide maximum water particleacceleration in the area between the transducer columns.

At the same time as the generator generates a pulse, it also generates asynchronizing pulse to a high voltage generator so that it delivers apowerful electric pulse but with a much shorter duration than theacoustic pulse.

This means that the acoustic and electric pulses are modulatedsynchronously. The fundamental frequencies for both types of pulse arein the infra level, i.e. below 20 Hz. The voltage level and the width ofthe electric pulse depend on the fish type, the size of the fish and thewater conductivity,.

The acoustic generator may preferably be adapted to generate drivevoltages for the transducers by:

i) providing a sine curve with the topical fundamental frequency,

ii) providing a number of odd harmonic curves with the sine curve as abasis, for example the first, third, fifth and seventh harmonies, and

iii) adding these odd harmonics to the sine curve with scalingcoefficients chosen from a knowledge of physical parameters of thetransducers, in order to create a periodic drive voltage curve with acharacteristic time behavior.

The characteristic time behavior of the drive voltage applied to eachtransducer results in an approximately square shaped time function forthe dynamic pressure in the water, and thus the highest possibleparticle velocity.

Preferably the generator is equipped to deliver the applied drivevoltage as well as the same applied drive voltage phase, to every othercolumn in the row, respectively.

The synchronization takes place with the aid of a simple square pulseplaced at the highest level of the acoustic pulse (at the leading edgethereof) which square phase at the same time triggers the electricpulse.

As an example, the grid can either be one-or two-dimensional, theone-dimensional grid comprising columns with only one transducer in eachcolumn, and the two-dimensional case comprising columns with two or moretransducers in each column.

In a special embodiment of the fish fence a regular additional row ofcolumns with only transducers and no electrodes can be used, i.e. thisadditional row is solely an infraacoustic row producing its own dipolefields.

Preferably, the transducer columns in the first row and in theadditional row are placed in two parallel lines, in a view from above,and in such a manner that each column in a row is placed at the summitof an equilateral triangle where two neighboring columns in the otherrow constitute the end points of the triangle base line, all in a viewfrom above.

The reason for a main row and an additional row (from the front, wherethe fish is to be barred) is that at the same time it is possible to usehydroacoustic detectors (of the echo sounder or sonar type) which detectfish approaching too near the fence.

Usually, only the acoustic part is switched on, i.e. only thetransducers are in operation, the electric part of the system beinginactive. When the fish approaches and enters the first row, it isdetected at the same time and the electric part of the main row at theback is activated. When the fish enters a first row, i.e. the additionalrow, the swimming speed is reduced, and it will show signs of fear. Whenit enters a second row i.e. the main row, it will also experience painfrom the electric pulse.

The signal level in the second row is preferably 3 dB higher than in thefirst row, i.e. the double.

By using "empty" areas, the two rows can be kept acoustically separated,so that the one only to a small extent influences the other.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed description of the now will be given of explanatory andnon-limitative examples, referring to the appended drawings, wherein:

FIG. 1A is a schematic view of a fence grid with transducers andelectrodes seen respectively from the side, indications of the acousticand electric fields also being shown.

FIG. 1B is a similar view from above,

FIG. 2 is a similar view a fence grid seen from above, with anadditional row consisting of acoustic transducers alone,

FIG. 3 is a section through an example of an electromagnetic lowfrequency transducer,

FIG. 4 is a schematic view showing what happens when a fish enters acombined acoustic/electric field (AE field)

FIG. 5 is a similar view showing overlapping AE fields with every secondpulse, and FIG. 6 is a graph showing curve shapes for acoustic andelectric pulses.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1, part A, shows an example of a fish fence according to thepresent invention. In such example four vertical columns are shown, eachwith three electromagnetic low frequency transducers 1 adapted toreceive signals from a not shown generator and distribution unit.

Additionally, FIG. 1A shows current electrodes 2, connection boxes 3 forthe current electrodes, floats 4 and anchoring on a bottom 5.

In FIG. 1B are shown dipole-shaped vector fields between transducers 1and between electrodes, where a dark arrow 7 indicates accelerationvectors and a lighter arrow 8 indicates electric field vectors which aremainly parallel to acoustic vectors. Around each transducer will appearpressure rings which are largest in one certain direction. Measurementsshow that the dipole field is relatively narrow, for example at adistance of two meters between each column the dipole field willpossibly extend 4-5 meters to each side, while a very intense fieldarises between the columns.

FIG. 2 shows a fish fence with an additional row in front of a main row,where the additional row is solely an acoustic row, while the main rowis an acoustic/electric row. In front of both rows there is ahydroacoustic detector 16 which is either of the sonar type or consistsof an echo sounder.

Eventually, a fish will enter the zone in front of the additional rowwhere it is detected, and consequently the electric field in the mainrow will be activated.

The reason that the electric field is not activated all the time is thatpower consumption can be reduced, and that it is desirable to reduce thedanger for mammals and humans who might enter the area of the field.

FIG. 3 shows a possible construction of a suitable transducer 1, with amembrane 11 beating up and down and constituting the bottom of thetransducer. As will be apparent from FIGS. 1A, 1B and 2, this willcreate strong particle velocity fields between the columns, as shownwith dark arrows. Additionally, the transducer has a top mounted fixingmeans 14 for cable and suspension, electronic circuitry (amplifier) 13,and a pressure compensated housing so that air 15 inside the transducerhas the same pressure as the water. An electromagnetic motor 12 drivesthe piston/membrane 11.

FIG. 4 graphically illustrates what happens when a fish 23 enters aparallel acoustic/electric field. The field is produced from atransducer 1 and an electrode 2 (corresponding elements can be found onthe right hand side, but are not marked by any reference numerals),these elements establishing the AE fields 25. The fish 23 will then withits side line organs sense the direction of the acoustic part of thefield 25, and at the same time feel pain and obtain muscle contractionsdue to voltage drop 24 (ΔV) across the fish, again due to the electricfield lines penetrating the fish body. The bigger the fish 23, thelarger the voltage drop 24 across the fish. This indicates that thevoltage, frequency and pulse width must be adjusted to the size of thefish.

Again, by changing these parameters, the fence can be adjusted to beeffective for fish above a certain size.

Furthermore, the intensity increases as one approaches the center linebetween the transducers. Around the transducers pressure circles 6 willbe created.

FIG. 5 shows very simply how to establish a relatively homogenous AEfield by using overlapping fields, i.e. every other two transducersoperate in pairs, and each of these pairs activates every other pulse.This means that transducer columns A and C cooperate in opposite phaseand thereafter transducer columns B and D operate in opposite phase, andthis takes place in successive order, every other time.

The ideal frequency is 5-7 Hz for each pair, the total frequency fromtwo pairs then being 10-14 Hz.

FIG. 6 shows the time function for the acoustic and electric pulses, fora main row and an additional row. Frequency and width of the pulses areindicated, but not their level.

I claim:
 1. A fish fence to be used under water based on low frequencymechanical vibrations in combination with a further stimulus, said fishfence comprising:a plurality of electromagnetic low frequencytransducers for mechanical vibrations, suspended in vertical columnsbeneath floats positioned in regular rows in or below the water surface,thus forming a transducer configuration constituting a grid; and acorresponding plurality of pairs of parallel-connected electricconductors, each conductor in a said pair of conductors being positionedclose to and along each respective side of a vertical transducer column,said further stimulus being an electric field from said conductors,modulated in synchronism with the mechanical vibrations from thetransducers.
 2. A fish fence according to claim 1, wherein saidtransducers in a column are arranged in oscillate in phase with eachother and in opposite phase with the transducers in the nearestneighboring column in the row, thereby to set up vibration fields as arow of single dipole configurations in the water between said columns oftransducers.
 3. A fish fence according to claim 1, wherein each saidtransducer is suspended for a downward acting movement of a vibratingmembrane of said transducer.
 4. A fish fence according to claim 1,wherein said transducers are connected to a generator for drivevoltages, the time behavior of which is adapted by means of a Fourieranalysis to give maximized water particle velocity in an area betweensaid columns of transducers.
 5. A fish fence according to claim 4,wherein said generator is adapted to generate drive voltages for saidtransducers byI) providing a sinusoidal curve with a topical fundamentalfrequency, II) providing a number of odd harmonic curves with afundamental sinusoidal curve as a basis, and III) adding said oddharmonic curves to said sinusoidal curve with scaling coefficientschosen from a knowledge of physical parameters of said transducers, tocreate a periodic drive voltage curve with a characteristic timebehavior, said characteristic time behavior of said applied drivevoltage on each transducer causing an approximate squarewave timefunction for dynamic pressure in the water outside membranes of saidtransducers, thereby providing maximum particle acceleration in thewater and hence the highest possible particle velocity.
 6. A fish fenceaccording to claim 5, wherein said odd harmonic curves comprise thefirst, third, fifth and seventh harmonic curves.
 7. A fish fenceaccording to claim 5, wherein said generator is adapted to deliver saidapplied drive voltage and the same applied drive voltage in oppositephase to respectively every other column in the row.
 8. A fish fenceaccording to claim 1, wherein said grid is in one dimension includingcolumns with only one transducer in each column.
 9. A fish fenceaccording to claim 1, wherein said grid is in two dimensions includingcolumns with at least two transducers in each column.
 10. A fish fenceaccording to claim 1, further comprising a regular additional row ofcolumns of transducers without associated electrode pairs, saidtransducers of said additional row being driven in a correspondingmanner and from a same generator as said transducers of a first regularrow, said additional row in addition to providing its own vibratingdipole fields also being adapted to generate further dipole fields bycooperating with said first row in an area between such two rows, saidtwo rows together forming a three-dimensional grid.
 11. A fish fenceaccording to claim 10, wherein said columns of transducers in said firstrow and in said additional row are laid out in two parallel lines whenviewed from above, and in such a manner that each column in one row ispositioned at the summit of an equilateral triangle where twoneighboring columns in the other row constitute end points of the baseline of said triangle, all viewed from above.
 12. A fish fence accordingto claim 1, wherein said electric conductors are positioned in such amanner and have a voltage source arranged in such a manner that theelectric field and a mechanical velocity vector field during synchronousoperation have substantially the same direction at any point in thewater at a distance from said columns and electrodes in the areastherebetween.
 13. A fish fence according to claim 12, wherein saidelectric field is constituted by a row of single dipole configurationsin the water between said columns of transducers/conductors.
 14. A fishfence according to claim 1, wherein said electric field is separatelyswitchable, independent of mechanical vibrations, as a response ondetection of incoming fish.
 15. A fish fence according to claim 1,wherein said electric field is active in a short timespan at a leadingedge of each periodic mechanical pulse from said transducers, a typicalpulse width being 25-75 ms at frequencies of 10-3 Hz, said timespantypically being 150-500 μs.